1. Field of the Invention
The invention relates to an imaging apparatus that is applied to a video camera capable of imaging at a low frame rate and to an imaging method that is applied to the imaging apparatus.
2. Description of the Related Art
A video camera, which obtains video signals conforming to a television broadcasting format or the like, employs a specified frame rate of 1/60 second, 1/50 second, or the like for capturing an image with an imager. For example, a video camera with a frame rate of 1/60 second has the maximum period (or shutter period) of 1/60 second for accumulating imaging light received with an imager for each frame.
When processing of shortening a period of receiving imaging light within one frame period (or electronic shutter processing) is carried out, an image can be captured at a high shutter speed such as 1/100 second, 1/1000 second, or the like. In this case, however, allowing a period of receiving imaging light to be longer than one frame period (or a low shutter speed) may not be carried out.
For realizing a shutter speed of lower than 1/60 second, imaging light for a long period over a plurality of frame periods can be received by an imager and then a signal corresponding to the imaging light is read out of the imager. For example, when a CCD (charge coupled device) image sensor is used as an imager, a period at which charges are accumulated in each pixel by receiving light can be extended. For example, when the electron-accumulation period for each pixel corresponds to the sum of two frame periods ( 1/30 second), a signal accumulated within 1/30 second can be read out. Thus, low-rate imaging with twice the accumulation period can be attained. The imaging signal thus obtained may be intermittently changed every 1/30 second that is twice the general frame period. Since the charge accumulation period is extended, imaging with high sensitivity can be attained as much. For example, an image can be captured even in darkness such as at night. In addition to the above-described exemplified processing of accumulating charges for twice the frame period, the period of accumulating electrons in the imager may be adjusted, for example, to allow the imaging to be extended for a period of several tens of frames.
Japanese Unexamined Patent Application Publication No. 2005-39710 discloses an example of an imaging apparatus that carries out processing of varying frame rates by frame addition.